sociated with lying and when a person is anxious excessive
amounts of blood flows to areas around the eyes. This blood
flow can be detected by facial thermal imaging. Another tech-
nique involves the usage of lasers to detect muscular, circu-
latory and other bodily changes associated with the anxiety
of lying. There is a method which uses the voice of a person
to detect whether he or she is telling the truth. Researchers
claim that the frequency of the voice when a person is lying
is different from the frequency produced when he is telling
the truth. When a person lies, an involuntary interference
of the nerves causes the vocal cords to produce a distorted
sound wave. A lie-detecting keyboard has been developed by
one company. This keyboard claims to identify liars by analyz-
ing typing patterns, sensing moisture in fingertips, recording
body heat and monitoring how fast the fingers are moving.
The P300 concealed information test tops all of the methods
mentioned before. An EEG (Electroencephalography) ma-
chine is used to run this test. This method uses the physiolog-
ical responses of the brain to detect liars. The basic working
principle of this machine is to provide the suspects with infor-
mation and if the information is meaningful then the brain will
send a unique signal wave called a P300 wave. For example,
if a subject is viewing a random series of names, one every
three seconds, and occasionally, one of these is the subject’s
name, a P300 wave is evoked in response to this rarely pre-
sented, recognized, meaningful stimulus. This is what makes
this test unique. By this, the police are able to detect sus-
pects who have prior knowledge of details that would be only
known by the suspects involved in the crime.
PAGE| 10
University of Peradeniya GAUGE Magazine
P300 lie detector only in-
volves an EEG machine and
requires only a few elec-
trodes to be pinned into the
test subject along with an
EEG cap. A typical P300 test
involves the following proce-
dure.
•Suspects are connected to
the EEG machine - one sus-
pect at a
time.
•A random collection of pic-
tures are shown which in-
cludes a single
photo involving the crime,
such as the murder weapon.
•The suspects are asked to
watch every photo with at-
tention.
•Brain-waves are observed
for possible P300 waves.
•Peak-to-peak value of P300
waves is observed.
•The suspect with the largest
P300 peak-to-peak value is
usually the
crimi-
nal. Only the actual criminal
would identify the murder
weapon
and create a
P300 irregularity.
The rates of correct detec-
tion in guilty and innocent
subjects in an experiment [1]
were 86%, which was better
than other previously men-
tioned methods. The effect
of countermeasures in the
test is extensively investigat-
ed by some researchers. The
results of such researchers
would ultimately result in a
more reliable P300 based lie
detector.
Science is improving in its